Global System for Mobile Communications

The Global System for Mobile Communications (GSM) is the most popular standard for mobile phones in the world. GSM service is used by over 2 billion people across more than 210 countries and territories [1] [2]. The ubiquity of the GSM standard makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs significantly from its predecessors in that both signaling and speech channels are Digital call quality, which means that it is considered a second generation (2G) mobile phone system. This fact has also meant that data communication was built into the system from very early on. GSM is an open standard which is currently developed by the 3rd Generation Partnership Project (3GPP).

The GSM logo is used to identify compatible handsets and equipment

From the point of view of the consumer, the key advantage of GSM systems has been higher digital voice quality and low cost alternatives to making calls such as text messaging. The advantage for network operators has been the ability to deploy equipment from different vendors because the open standard allows easy inter-operability. Like other cellular standards GSM allows network operators to offer roaming services which mean subscribers can use their phones all over the world.

As the GSM standard continued to develop, it retained backward compatibility with the original GSM phones; for example, packet data capabilities were added in the Release '97 version of the standard, by means of GPRS. Higher speed data transmission has also been introduced with EDGE in the Release '99 version of the standard.

History

Throughout the evolution of cellular telecommunications, various systems were developed without the benefit of standardized specifications. This presented many problems directly related to compatibility, especially with the development of digital radio technology. In 1982, The GSM group ("Groupe Spécial Mobile" (French) 1, 2, 3 and 4) was formed to address these problems. The name of the system comes from the name of this group, though later the decision was made to keep the initials but to change what they stood for. Originally the group was hosted by European Conference of Postal and Telecommunications Administrations (CEPT).

From 1982 to 1985 discussions were held to decide between building an analog or digital system. After multiple field tests, a digital system was adopted for GSM. The next task was to decide between a narrow or broadband solution. In May 1987, the narrowband time division multiple access (TDMA) solution was chosen.

By the mid-1980s, many of the European countries had developed their own systems. This led to disagreement on what system to use. The conflict almost stopped the project. However, the European Union intervened and all 15 countries decided to choose the standard recommended by CEPT. In February 1987, eight different systems were tested in a competition held in Paris. A system developed by researchers Torleiv Maseng and Odd Trandem at the Norwegian University of Science and Technology won and was chosen. This was particularly surprising as they had no industry backing. Their system won by its ability to counter signal reflection in urban areas and in mountainous terrain.

The technical fundamentals of the GSM system were defined in 1987. In 1989, the European Telecommunications Standards Institute (ETSI) took over control and by 1990 the first GSM specification was completed, amounting to over 6,000 pages of text. Commercial operation began in 1991 with Radiolinja in Finland.

In 1998, the 3rd Generation Partnership Project (3GPP) was formed. Originally, it was intended only to produce the specifications of the next (third, 3G) generation of mobile networks. However, 3GPP also took over the maintenance and development of the GSM specification. ETSI is a partner in 3GPP.

GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to not limit the designers yet still make it possible for the operators to buy equipment from multiple suppliers.

Radio interface

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including the United States and Canada) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated.

The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.

In the 900 MHz band the uplink frequency band is 890-915 MHz, and the downlink frequency band is 935-960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. Time division multiplexing is used to allow eight speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.

The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.

GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbit/s.

There are four different cell sizes in a GSM network - macro, micro, pico and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose diameter is a few dozen meters; they are mainly used indoors. On the other hand, umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.

Cell radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is 35 km or 22 miles. There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.

Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors, for example in shopping centers or airports. However, this is not a pre-requisite, since indoor coverage is also provided by in-building penetration of the radio signals from nearby cells.

The modulation used in GSM is Gaussian minimum shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent channel interference).

Network structure

The structure of a GSM network

The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
* the Base Station Subsystem (the base stations and their controllers).
* the Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
* the GPRS Core Network (the optional part which allows packet based Internet connections).
* all of the elements in the system combine to produce many GSM services such as voice calls and SMS.

Subscriber identity module

One of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phonebook. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries.

In the United States, Europe and Australia, many operators lock the mobiles they sell. This is done because the price of the mobile phone is typically subsidised with revenue from subscriptions and operators want to try to avoid subsidising competitor's mobiles. A subscriber can usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. While most web sites offer the unlocking for a fee, some do it for free. The locking applies to the handset, identified by its International Mobile Equipment Identity (IMEI) number, not to the account (which is identified by the SIM card). It is always possible to switch to another (non-locked) handset.

Some providers in the USA and Europe, such as T-Mobile, Cingular and the three French Operators, will unlock the phone for free if the customer has held an account for a certain period. Third party unlocking services exist that are often quicker and lower cost than that of the operator. In most countries removing the lock is legal.

A curious exception to this rule is Belgium, where all phones are sold unlocked. However, it is unlawful for operators there to offer any form of subsidy on the phone's price. This was also the case in Finland until April 1 2006, when selling subsidized combinations of handsets and accounts became legal though operators have to unlock phone free of charge after a certain amount of time (at most 24 months).

GSM security

GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using shared-secret cryptography. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticated the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation.

GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. A large security advantage of GSM over earlier systems is that the Ki, the crypto variable stored on the SIM card that is the key to any GSM ciphering algorithm, is never sent over the air interface. Serious weaknesses have been found in both algorithms, and it is possible to break A5/2 in real-time in a ciphertext-only attack. The system supports multiple algorithms so operators may replace that cipher with a stronger one.

Patent issues

In 2005, a number of companies Koshie (including Cisco Systems and Ericsson) were sued for infringement of U.S. Patent No. 5,561,706, for offering products alleged to be compliant with the GSM 3.60 standard.

External links

* GSM Association - the group representing GSM operators (official site) - includes coverage maps for all members
* 3GPP The current standardisation body for GSM with free standards available.
* 3GPP numbering scheme
* GSM/Telecom Resources
* Overview of GSM by John Scourias
* Visualtron's tutorial on GSM
* GSM technical overview and tutorial from Radio-Electronics.Com
* Number of GSM Subscribers
* GSM-security.net FAQ
* GSM Call Flow Diagrams
* List of acronyms of GSM network parameters
* GSM - ideas, origin and milestones - a Norwegian perspective from Telenor's journal of technology Telektronikk
* Cell Phone Safety and Wireless Facts
* GSM Wireless Terminals
* Smart Car Surveillance System How to trace your missing car using a GSM cell phone (Wiki eHow)
* Linkbit Online GSM Message Decoder

Global System for Mobile Communications: Encyclopedia BETA

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